1. Field of the Invention
Exemplary embodiments of the present invention relate to a low noise amplifier which is used in a receiver stage of a fourth-generation mobile communication terminal system; and, more particularly, to a controlled-gain wideband feedback low noise amplifier which operates in a high-gain mode, a mid-gain mode, or a low-gain mode, depending on a control voltage.
2. Description of Related Art
In general, a signal received by a radio frequency (RF) receiver stage has a very low power level due to the influence of attenuation and noise. Therefore, it is necessary to amplify the power of the received signal. Since the received signal contains much noise while being transmitted through a wireless communication channel, an amplification function for minimizing the noise is required.
The RF receiver stage is a first-stage system of a receiver, which may affect the performance of the receiver. The RF receiver stage typically includes a low-noise amplification unit, a mixer, and a local oscillator. To configure a high-performance receiver system, a noise factor (NF) and amplification gain of the low-noise amplification unit, a conversion loss and harmonic distortion of the mixer, and the frequency stability of the local oscillator should be considered. However, the NF and amplification gain of the low-noise amplification unit may have the largest effect upon the performance of the system.
A low-noise amplifier (LNA) is a high-frequency amplifier (LNA) configured to reduce the NF of the entire receiver including the low-noise amplification unit. The LNA is used for a communication line with a large propagation loss or the transmission and reception of signals with a low input voltage, and serves to amplify a weak signal received by an antenna in the communication system.
To obtain a low-noise characteristic, a small amount of current should be used while a small number of thermal noise elements such as transistors and resistors having a low NF are used. Furthermore, a maximum gain needs to be secured through matching.
FIG. 1 is a circuit diagram of a conventional LNA. Referring to FIG. 1, the conventional LNA will be described as follows. The conventional LNA has a cascode structure in which a common-source transistor Q1 and a common-gate transistor Q2 are connected in series to each other.
In the conventional LNA illustrated in FIG. 1, a resistor R5 and a capacitor C5 are connected in series to each other such that the output of the common-source transistor Q1 stably operates. Furthermore, the output of the common-gate transistor Q2 is connected to an input terminal through a feedback structure of a shunt-feedback scheme so as to control a gain of the LNA without changing input impedance of the common-source transistor Q1.
However, the conventional LNA is not provided with a controlled-gain function. Therefore, to support the controlled-gain function, a controlled-gain amplifier should be used together with the conventional LNA. Accordingly, the conventional LAN with a controlled-gain amplifier is not suitable for being used in a receiver stage of the fourth-generation mobile communication terminal system of which the utilization space and power consumption should be minimized mechanically and functionally.